Manganese redox cycling in Lake Imandra: impact on nitrogen and the trace metal sediment record

Abstract. Sediment and water samples from the mine-polluted Yokostrovskaya basin in Lake Imandra have been analysed. Three major processes have influenced the accumulation and distribution of metals in the sediment: (1) Development of the apatite-nepheline and the sulfide ore mining industries. (2) Secondary formation of sulphides in the upper sediment column. (3) Redox cycling of Mn in the surface sediment and in the bottom water.

This study demonstrate the dominant role of the Mn redox cycling in controlling distribution of several major and trace elements, especially during the winter stratification period. Mn oxides act as a major scavenger and carrier for the non-detrital fraction of Al, Ca, K, Mg, P, Ba, Co, Cu, Ni, Mo and Zn in the bottom water. Aluminium, Ca, K, Mg, P, Cu, Ni and Zn are mainly sorbed at the surface of the particulate Mn phase, while Ba and Mo form a phase (or inner sphere complex) with Mn. Co is associated with the Mn-rich phase, probably by oxidation of Co(II) to a trivalent state by the particulate Mn surface. Formation and dissolution of Mn particles most likely also control anoxic ammonium oxidation to nitrate and reduction of nitrate to N₂. It is shown that secondary sulphides in Lake Imandra sediments are fed with trace metals primarily scavenged from the dissolved phase in the water column. This enrichment process, driven by the Mn-redox cycle, therefore changes the sediment record by the transfer of a dissolved pollution signal to the particulate sediment record, thus making it more complicated to trace direct influence of particles from different pollution sources.